Apparatus for controlled switching of a microcomputer to standby mode

ABSTRACT

A voltage regulator connected to a microcomputer is switched from main power to standby power by a monitoring integrated circuit in response to a switch off by the control circuit after receiving a switch off command, from an ignition switch, and only after the computer has been reset. The monitoring integrated circuit has a plurality of circuit elements including a circuit element which generates a control signal for switching the voltage regulator to a state producing a standby voltage which powers the microprocessor to save the information in the microprocessor memory. Preferably the circuit element generating the control signal is an operational amplifier whose output is fed by a time delay network which delays switching of the voltage regulator.

BACKGROUND OF THE INVENTION

The present invention relates to circuitry for switching a microcomputerto a standby mode once the microcomputer is in a defined state, normallya reset state.

It is known that, if one wishes to retain information in a random accessmemory (RAM), it is necessary to provide a standby power supplyconnected to the RAM even when an associated microprocessor is switchedoff. Care must be taken, however, to ensure that the informationcontained in the RAM is not changed when the microcomputer is switchedoff in the middle of accessing a RAM.

Various circuits have been proposed for achieving a controlledswitch-over to standby supply for a RAM. Unfortunately, many of thesecircuits utilize a number of components and occupy a relatively largeamount of space. Until now, this has rendered them inappropriate forincorporation in hybrid devices used for multicircuit electronicequipment used in motor vehicles.

U.S. Pat. No. 4,512,019 describes a circuit for monitoring amicrocomputer and, in particular, a circuit for resetting themicrocomputer when a malfunction occurs or when the supply voltage dropsto a value below a predetermined level.

In Patent Abstracts of Japan, Vol. 9, No. 214, p. 384 and JP 60-73 720 adevice for preventing the destruction of stored data in a RAM isdescribed. In this device the base of a switching transistor installedbetween the main power source and the power input terminal of the RAM isconnected to the output terminal of a voltage-drop detecting element.

A device for maintaining operation as long as the voltage fluctuationsof a main power source are within a predetermined range of recommendedworking source voltages of a CMOS-RAM is described in Patent Abstractsof Japan, Vol. 10, No. 389, P. 531 & JP 61-177 523. Therefore a relaycontact for switching between the main power source and a chargeabletype secondary backup battery is proposed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus forswitching a random access memory of a microcomputer to standby power.

The present invention provides a power supply arrangement for amicrocomputer comprising regulator means for supplying a regulated mainvoltage or a regulated standby voltage to a microcomputer, and amonitoring integrated circuit having a plurality of circuit elementswhich is connected to the microcomputer for sending a resetting signalto the microcomputer. A circuit element of the monitoring integratedcircuit has an output connected to a control input of the regulatormeans via a threshold circuit. The monitoring integrated circuit isarranged to generate, in response to a resetting signal after apredetermined time delay, a control signal for switching the regulatormeans when the microcomputer has reached a defined state for switchingto a standby mode. The advantage of the above-described arrangement isthat an existing circuit can be utilized without any redesign of thecircuit itself.

In more detail, the voltage regulator means has a control input forreceiving an "on" signal to switch the voltage regulator means to supplythe regulated main power to the microcomputer, but when the "on" signalis not present at the control input the regulated standby power issupplied. The "on" signal is delivered by means for providing the "on"signal which include an ignition switch so that, when the ignitionswitch is switched, the "on" signal is provided at the control input andthe voltage regulator means delivers the main power to themicrocomputer. The monitoring integrated circuit provides a resettingsignal in response to the "on" signal which is also provided to it at amonitoring input. A time delay network including a capacitor andresistors is provided connected to the control input and to themonitoring integrated circuit. This time delay network provides a timeinterval during which the "on" signal is maintained at the control inputof the voltage regulator means after switching the ignition switch toprovide the standby power.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the present invention will nowbe illustrated in more detail by the following detailed description,reference being made to the accompanying drawing in which:

The sole FIGURE is a circuit diagram of a preferred embodiment of apower supply arrangement for a microcomputer according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is based on an existing integrated circuitrepresented generally by the reference numeral 10, which comprises anumber of operational amplifiers and components of a "watchdog circuit".

The structure and operation of the circuit 10 is described in moredetail hereinbelow.

Associated with circuit 10 is a microcomputer 11 supplied with powerfrom a voltage regulator 12 which, in turn, is connected via an inputterminal 13 to the battery B of the vehicle. The voltage regulator 12supplies two regulated voltages: a main regulated power supply voltageV_(out) from the V_(out) output terminal 12a during normal operation,and standby power supply voltage Stby from the output terminal 12b. Thestandby power supply is used to retain information in RAM R associatedwith the microcomputer 11. As is known, operation of the ignition switchI connected to terminal 15 is detected and a signal supplied to an ONinput 12c which causes the voltage regulator to switch ON or OFF.

In the present embodiment, the terminal 15 connected to the ignitionswitch I is connected to the terminal 12c of the voltage regulator viadiodes D2, D3 and resistor R4. A diode D1 is inserted in the connectionbetween the terminal 13 and the main power supply to the regulator 12.The diodes D1 and D2 are used as polarity reversal protection diodes.

With the ignition switch I in its ON condition, a voltage signal ispresent at the terminal 15 and the voltage regulator 12 is in its normaloperating condition supplying power to the microcomputer 11 from theoutput terminal 12a. A signal is also present on the VE input of thecircuit 10 via resistor R2 which is connected to the output of the diodeD2. The input terminal VE is the noninverting input of an operationalamplifier 01 whose output is taken from the terminal VA and fed to theinput terminal KE of an additional operational amplifier 02 having anopen collector output KA. The output KA is of high impedance and enablesvia line 10a the watchdog circuit containing a part of the integratedcircuit 10 and external components R6, R7, C3. The watchdog circuit issupplied with a watchdog signal WD shown with the dashed line in theFIGURE from the microcomputer 11 by which it monitors operation of themicrocomputer and provides a reset signal in the event of a malfunction.It also provides a reset signal in response to a change in potential atinput S. Thus a change in potential at the input S is reflected by achange in output potential at the output R (reset) which is connected tothe microcomputer 11. The watchdog circuit may be a monitoring circuitof the type described in U.S. Pat. No 4,512,019 assigned to the sameassignee. In normal operation, with the ignition switch closed, thepotential at input S is high which means that the output at R is low andthe microcomputer is not reset. The level of the signal at output R isfed to the inverting input XE of an additional operation amplifier 03whose output XA is connected via a time delay network of resistors R5and R4 and capacitor C1 to the control terminal 12c of the voltageregulator 12. Diode D3 decouples the output XA from the input VE.

When the ignition switch I is switched OFF. the voltage on terminal 15disappears and after a capacitor C2 connected to the input terminal VEhas discharged below a low triggering threshold, the output VA switchesto a low potential which, in turn, switches the output KA to a lowpotential to turn off the watchdog circuit by lowering the potential atinput S. In turn, the output R goes high which resets the microcomputer11 and also provides a high potential input to the terminal XE. Becausethe amplifier connected between XE and XA is inverting, the potential atoutput XA goes low and this in turn causes the potential at 12c to dropto a low potential after a capacitor C1 of the time delay networkconnected thereto has discharged and the microcomputer has reached adefined state for switching to standby mode. A low potential at input12c causes a first transistor in the regulator 12 to switch on thusremoving voltage from the base of a main transistor and turning it off.This causes the voltage regulator 12 to switch to provide a standbyvoltage on the terminal 12b to maintain the RAM R with standby power.

It will be appreciated that the above arrangement can be modified fordifferent types of monitoring integrated circuits 10. For example, theflip-flop between terminals S and R could provide a noninverted outputwhich would result in a change for the amplifier connected betweenterminals XE and XA to be a noninverting amplifier. Further, asindicated by the dashed line the microcomputer itself, by detecting alow voltage at VA, can provide the necessary watchdog signal to causethe monitoring integrated circuit 10 to generate the appropriate changein potential at the terminal 12c of the voltage regulator to switch thevoltage regulator to a standby mode.

I claim:
 1. A power supply arrangement for a microcomputer having anassociated random access memory means, comprising a battery, voltageregulator means for producing a regulated main power at a main voltageand a regulated standby power, said voltage regulator means beingconnected to a microcomputer having a random access memory to supply theregulated main power to the microcomputer when the microcomputer isswitched on and the regulated standby power to the microcomputer toretain information in said random access memory when the microcomputeris switched off, and said voltage regulator means having a control inputfor receiving an "on" signal to switch the voltage regulator means tosupply the regulated main power to the microcomputer, said voltageregulator means supplying said regulated standby power when said"on"signal is not present at said control input;means for providing the"on" signal connected electrically to the control input, said means forproviding including an ignition switch; a monitoring integrated circuitmeans for producing a resetting signal to the microcomputer connectedelectrically to the microcomputer, said monitoring integrated circuitmeans having a monitoring input connected electrically to said means forproviding the "on" signal and producing a resetting signal in responseto an "on" signal received at said monitoring input; a time delaynetwork including a capacitor and resistors connected to the monitoringintegrated circuit means to maintain the "on" signal at said controlinput for a time interval after the ignition switch has been switchedoff to trigger the voltage regulator means to supply the regulatedstandby power to the microcomputer so that the voltage regulator meanscontinues to supply the regulated main power for the time interval, saidtime interval being of a sufficient duration so that the microcomputerhas reached a defined state in response to the resetting signal producedby the monitoring integrated circuit means before said voltage regulatormeans switches to provide said regulated standby power to saidmicrocomputer.
 2. A power supply arrangement according to claim 1,wherein said monitoring integrated circuit means has a safety input forreceiving a self-monitoring signal from the microcomputer to reset themicrocomputer when the microcomputer malfunctions.
 3. A power supplyarrangement for a microcomputer having an associated random accessmemory means, comprising a battery, voltage regulator means forproducing a regulated main power at a main voltage and a regulatedstandby power, said voltage regulator means being connected to amicrocomputer having a random access memory to supply the regulated mainpower to the microcomputer when the microcomputer is switched on and theregulated standby power to the microcomputer to retain information insaid random access memory when the microcomputer is switched off, andsaid voltage regulator means having a control input for receiving an"on" signal to switch the voltage regulator means to supply theregulated main power to the microcomputer, said voltage regulator meanssupplying said regulated standby power when said "on" signal is notpresent at said control input;means for providing the "on" signalconnected electrically to the control input, said means for providingincluding an ignition switch; a monitoring integrated circuit means forproducing a resetting signal to the microcomputer connected electricallyto the microcomputer, said monitoring integrated circuit means having amonitoring input connected electrically to said means for providing the"on" signal and producing a resetting signal in response to an "on"signal received at said monitoring input; a watchdog circuit forproducing the resetting signal, the monitoring integrated circuit meansincluding two operational amplifiers connected electrically in seriesbetween the monitoring input and the watchdog circuit so that theresetting signal is generated in response to a change in potential atthe monitoring input as a result of receiving the "on" signal; and atime delay network including a capacitor and resistors connected to themonitoring integrated circuit means to maintain the "on" signal at saidcontrol input for a time interval after the ignition switch has beenswitched off to trigger the voltage regulator means to supply theregulated standby power to the microcomputer so that the voltageregulator means continues to supply the regulated main power for thetime interval, said time interval being of a sufficient duration so thatthe microcomputer has reached a defined state in response to theresetting signal produced by the monitoring integrated circuit meansbefore said voltage regulator means switches to provide said regulatedstandby power to said microcomputer.
 4. A power supply arrangement asdefined in claim 3, wherein the time delay network comprises a circuitconsisting of the capacitor and one of the resistors, said circuit beingconnected electrically in series between the control input and themonitoring integrated circuit means.